1. Field of the Invention
This invention relates generally to magnetic tape systems and more particularly to a magnetic tape cartridge including a magnetic tape track position indicator attached thereto.
2. Description of the Prior Art
Various systems have been taught in the prior art to provide for positioning a magnetic head having a single track along the width of a magnetic tape to allow the recording and reading of a plurality of tracks longitudinally recorded on the magnetic tape. U.S. Pat. No. 3,859,662 discloses a transducer track selection apparatus wherein precise positioning of a magnetic head is provided by a toothed rack which cooperates with a single tooth on the magnetic head carrier. The toothed rack, which includes a plurality of teeth each representing a particular recording track position, is included with, and mounted on, the track selection apparatus. Thus, if a tape were recorded on one reader/recorder apparatus and moved to another reader/recorder using a track selection apparatus of the same design, repeatability in accessing appropriate tracks would not be insured, since there could be some variation between the magnetic head position and previously recorded tracks of data on the tape. This variation could occur because of slight differences in the dimensions of the racks at the time of their manufacture, because of differences in the mounting of the racks on the track selection portions of the reader/recorders or because of variations in the orientation of the tape with respect to the reader/recorder.
In the above track selection apparatus it is relatively easy to assure accuracy and repeatability of track accessing when the tracks are relatively wide. In this case, relatively considerable differences could occur between the manufactured dimensions of the toothed rack, or between the mounted positions of the racks, while repeatability and track accessing could be assured. However, as the tracks are made narrower to allow the recording of more tracks along a given width of magnetic tape, the problems of accuracy and repeatability of track accessing become more severe. This is particularly true when it is required that the tapes be changed from one machine to another between reading and recording.
Another problem with the above-described system is one of wear in the track selection apparatus. Obviously, the requirement that a single tooth mechanically engage a toothed rack for each track selection operation causes some degree of wear in the track selection operation that could require the eventual replacement of parts in the apparatus when a large number of track selection operations have been performed. It is known in the prior art to employ a phototransducer assembly in conjunction with a marked, optical mask to provide positioning indications in magnetic head track selection operations. U.S. Pat. No. 3,839,664 is an example of this teaching. However, although the wear problem can be substantially eliminated by utilization of the phototransducer-optical mask system, the consideration of assuring accuracy and repeatability of track accessing when relatively narrow recording tracks are employed and when it is necessary to record the tape on one system and play back the tape on a similar system cannot be ignored. That is, differences in the dimensions of the mask at the time of manufacture or differences in the precise mounting of the mask from one system to another certainly affect accuracy and repeatability of track selection.
In neither of the above systems is any mention made of expansion of the track position indicator (toothed rack or optical mask) because of temperature and/or humidity changes. As track widths become narrower and narrower and more and more closely packed together, significant expansion of the track position indicator and/or the magnetic tape can seriously affect the accuracy or repeatability of selecting a particular, desired track.
It would, therefore, be advantageous to provide reliable track positioning from one system to another, regardless of temperature and/or humidity variations, of a single track magnetic head relative to a multiple track magnetic tape.